The present invention relates to a frequency counter for counting a signal frequency and particularly for counting in real time the signal frequencies partly varying as time passes.
In recent science and engineering field, there are many cases that significant information is obtained from frequency variation of a signal. For example, in ultrasonics, frequencies of echo signals obtained by reflected ultrasound waves from targets located along the transmission direction of a bursted ultrasound wave are varied because of a characteristic of respective target. In other words, the characteristic of respective target can be studied by counting the frequency variation of the ultrasound echo signal from respective target. The same advantage can be obtained in other cases such as a case of Doppler effect application. As well known, in the case of Doppler frequency, the frequency variation gives the information of the movement of a target.
In such cases, if the frequency variation can be counted in real time, more advantage could be obtained. For instance, in the case of the ultrasonics, if the frequency variation due to the target characteristic is counted in real time, quick target response to, for example, some action added to the target may be analyzed, and from which new information of the target characteristic could be studied. In the case of Doppler effect, the quick movement of the target can be studied more precisely. However, in the prior art, it has been not easy to count the frequency variation in real time.
FIG. 1 is a block diagram of the frequency counter of the prior art, and typical waveforms are shown in FIGS. 2(a), 2(b) and 2(c) respectively to the signals having reference letters A, B and C in FIG. 1. In FIG. 1, an input signal A received at an input terminal 1 is led to a zero-cross detecting circuit 2 and detected therein so that a zero-cross signal B shown in FIG. 2(b) is output. The zero-cross signal B is led to a counter 3 in which the number of pulses of the zero-cross signal B is counted during a time interval .DELTA.t. Before counting the pulses, the counter 3 is cleared every time when a synchronizing signal C, generated at a synchronizing signal generator 4, is applied to the counter 3, and counts the number of pulses until the counter 3 is cleared by the next synchronizing signal C. The synchronizing signal is a train of positive narrow pulses having a time period .DELTA.T. Wherein, the interval .DELTA.t is equal to a time interval from the trailing edge of a synchronizing pulse to the leading edge of the succeeding synchronizing pulse. The output signal from the counter 3 is led to a latch circuit 5 in which the output signal is latched until the counter 3 is cleared by the succeeding clear pulse, so that the numbers of the zero-cross signals in every interval .DELTA.t is output from an output terminal 6.
As seen from the above, in the prior art, the frequency counter counts the number of the zero-cross signals in the interval .DELTA.t. Therefore, there is a problem in the prior art that it is impossible to count the number (in other words, the frequency) partly varied within the interval .DELTA.t. Of cause, it is impossible to count it in real time.
If a partly varied frequency of the zero-cross signal in the interval .DELTA.t is attempted to be forcibly counted by the prior art frequency counter, whereby the period .DELTA.T must be decreased to, for example, .DELTA.T' so that the varied frequency can be partly counted. However, in this case, frequency counters as many as corresponding numbers of .DELTA.T/.DELTA.T' must be prepared which is realistically difficult and has been a significant problem in the prior art.